It's called tinnitus. Usually it comes and goes, could last forever.
I had it bad after a buddy of mine shot over me with a 20ga so I went to get my ears checked. Everything checked out fine and I still had great hearing.
I usually don't wear ear protection while hunting, although I need to start getting into the habit of it, at least some of the time. What really matters though that a lot of people don't think about is other loud machinery. The boat engine on the way to the blind/fishing hole, lawn mower, weed eater, etc. All these things do damage to your hearing even if it's only a little. It also does not help with tinnitus.
I've found that if the ringing is really bad, sometimes I can put an earplug in that ear for a while and it helps, but there's really not a whole lot you can do other than learn to ignore it.
Also, thanks for making me aware of the ringing in my right ear that I've been succesfully ignoring all day!